1. Field of the Invention
This invention relates to a method of preparing high purity benzoic acid at a high yield from crude benzoic acids obtained by individual oxidation of toluene. The invention is also concerned with the benzoic acid prepared thereby.
In catalytically oxidizing toluene with air to produce benzoic acid, byproducts of various nature are coproduced, such as benzaldehyde, benzyl alcohol, diphenyl, 2- and 3-methyldiphenyl, benzylbenzoate, diphenylcarboxylic acids, etc. Such byproducts accompany the toluene and benzoic acid leaving the oxidation reactor and must be separated if a suitable benzoic acid for industrial and pharmaceutical applications is to be obtained.
2. Prior Art
Toluene oxidation to prepare benzoic acid, and purification of the latter, is disclosed for example in UK Pat. No. 1,219,453 and French Pat. No. 1,560,868. In essence, such prior processes are based on oxidation of toluene with pressurized air at 6-15 atmospheres and 150.degree. to 180.degree. C., and in the presence of suitable catalysts, such as Co-Mn, on distillation first of the non-converted toluene which accompanies the benzoic aicd, followed by purification by rectification of the benzoic acid in a tray column, wherein benzaldehyde, benzyl alcohol, toluene left-overs, and other byproducts still present in minor amounts separate as overheads; this stream, which contains appreciable amounts of benzoic acid, is usually cycled back to the oxidation reactor.
With other conventional methods, benzaldehyde is separated from the stream, as by stripping and rectifying arrangements, while the volatile fraction and residue from this separation are returned to the oxidation reactor. The end products from primary rectification of benzoic acid are further distilled with a secondary rectification to obtain benzoic acid, albeit at a strength not exceeding 80%, which is passed to the oxidation reactor. The residue from this second distillation, which contains the high-boiling impurities, is burned off. From the primary rectification column benzoic acid is yielded at a strength of approximately 99%. Impurities may have on the average the following composition: diphenyl, 2- and 3-methyldiphenyl, etc.
When operating in accordance with the above prior art, such cycling of the overhead products from primary rectification (or of overhead and end products after benzaldehyde has been recovered) and secondary rectification distillate back to the reactor has two main disadvantages, namely:
(1) The content in byproducts of the streams flowing from the oxidation reactor to the primary rectification column is increased, until such byproducts find their outlet either through the end products from the second rectification, or secondary rectification, or production; this affects the quality of the resulting benzoic acid; PA1 (2) Decreased capacity of the oxidation reactor; in fact, the rate of oxidation of toluene with a soluble catalyst and air is inversely proportional to the concentration of benzoic acid in the oxidation reactor toluene; for example, with benzoic acid concentrations in excess of 35%, and operating at temperatures in the 155.degree. to 180.degree. C. range and pressures in the order of 6-15 atmospheres, reaction stops. Thus, cycling large amounts of benzoic acid back to the oxidation reactor results in a decreased output. PA1 (a) at least one distillation and at least one crystallization in an organic solvent; PA1 (b) scrubbing of the crystals from (a) using the same solvent as in (a); PA1 (c) cycling of the mother liquors from the crystallization as in (a) plus the mother liquors from (b) back into the dissolver; PA1 (d) further scrubbing of the crystals from (b) using the same solvent as in (a), the scrubbing liquors being cycled back; and optionally PA1 (d1) further scrubbing of the crystals as scrubbed in (d) with water.